They are interested. in the structure of the ACDS (Acetyl-CoA Decarbonylase SYnd'ase) multienzyme complex which is responsible for the synthesis and cleavage of acetyl-CoA in methanogens. To determine the detailed oligorneric structure of the complex ( -1.6-2 MDA), accurate molecular masses are needed. ACDS complexes from Methanosarcina therinophila and M. barkeri contain five different subunits designated -a, P, y, 6, and F- The complex can also be dissociated into three protein subcomponents, one of which is (a + e). It is thought that the (a+e) particle is a heterotetramer (oc2e2) of approximately 220 kDa, but gel filtration data have indicated a value of 350 kDa. To start this project, fi-eeze-dried specimens of the (a+c:) particle were prepared. Preliminary ma s measurements indicate that it is a heterotetramer. The (y + 8) subcomponent from the complex has also been examined in the STEM for mass analysis, which is being performed now.